The present invention is related to the packaging of semiconductor devices. In particular, the present invention is related to the method and apparatus for the removal of mold flashes found in integrated circuit (IC) devices packages using laser technology.
The packaging of semiconductor and IC devices by plastic molding of the chip is well known in the art. The package typically contains a molded plastic covering which protects the IC devices inside. A metallic heat sink exposed on the surface of the plastic packaging allows efficient heat transfer between the IC device and the exterior environment. Leads extending from the packages allow electrical connection between the device and the external circuitry.
During the packaging process, the IC device, with the heat sink and the leads, is retained in a mold wherein molten plastic is injected. When the molten plastic is solidified, the IC chip becomes embedded in the plastic packaging. The metallic heat sink ideally should be free of any excess molding material known as mold flash. However, due irregularities in the mold, mold flash is inevitably bled onto the edges of the heat sink.
Traditional ways of removing mold flash from IC packages, such as sand blasting, water jetting and chemical etching, can easily damage or affect the reliability of the packaging. Recently laser-induced removal of mold flash has been suggested.
Singapore patent 49545 describes the use of short wavelength pulse laser for the removal of mold flash. In this prior art document, a short wavelength laser is used as a heat source and a momentum source for the removal of the mold flash. Using this method, the region with mold flash, such as the interface between the leads and the lead frame itself, is heated up. Thermal expansion of the lead frame occurs, and due to the difference between the expansion coefficient of the lead frame metal and that of the mold flash, the lead frame metal expands more than the mold flash. In addition, the laser beam as a momentum source, which further destroys any bonding between the metallic lead surface and the mold flash. These two actions results in the cleavage of the mold flash from the lead frame. This method, however, cannot be directly applied to the deflashing of the heat sink of the IC devices, since thermal expansion of the heat sink is not desirable or useful in the deflashing process.
In other instances, laser is used in a thermal melting process, in which the flash is melted and vaporized. Although the mold flash may be removed in this way, the high heat intensity generated by this method causes traces of melting on the metal surface of the heat sink, which can produce micro-cracks and other damages. As a result, there is a need to search for other techniques which are effective and gentle on the IC device.
It is therefore an object of the present invention to provide a method and apparatus for laser removal of mold flash which overcome the shortcomings as stated above.
It is another object to provide one embodiment of a laser apparatus which has high efficiency but minimizes damages to the packaging itself.
Other objects and features of the invention will become apparent to those skilled In the art as the disclosure is made in the following detailed description of the preferred embodiments as illustrating in the accompanying sheets of drawings.
The present invention is a method and apparatus for the removal of mold flash from an IC device using a laser ablation method. Ablation is achieved under conditions in which the mold flash is converted to plasma under short laser pulses which do not give sufficient time or energy for significant thermal processes to occur. As a result, the metallic portion adjacent the mold flash, such as the heat sink underneath, is prevented from heating up due to lack of heat transfer, thereby protecting the metallic surface and the interior die from heat damages. According to one embodiment of the invention, a mask is provided to protect the molded packaging of the IC device from the laser beam. The mask may have at least one hole which corresponds to the heat sink of the device wherethrough the laser beam can pass. According to another embodiment of the invention, a large beam diameter is provided to increase the efficiency of the deflashing process.
In the preferred embodiment, the fluence of the laser beam is kept low at below 1J/cm2, the wavelength of the laser used is below 550 nm, while the pulse width of the beam is kept between 8-10 ns. With a beam spot diameter of 12-15 mm, deflashing by photodecomposition can be effective performed. Together with the mask, entire strips of IC devices may be deflashed by scanning the beam across the strip.